There's a few.
A) Superliminal Flight + Rocket Charge or Rocket Fists (the first two are from Powers Unlimited 3, the latter from PU 2). Get going at Superliminal. Unlike Cosmo-Knight flight, this is NOT a hyperspatial jump, but your character going CONTINUOUSLY at that speed for the whole arc. Rocket Charge and Rocket Fists make it so you ignore ramming damage. Page 72 of HU Revised 2nd Edition gives you +4 damage per mile per hour. So:
186,000 miles per second per level [since you get x1 FTL per level] * 60 seconds per minute * 60 minutes per hour * 4 damage = Your base striking damage + 2,678,400,000
Since they imply heavily (though don't explain since they do a terrible job at details like this) that this damage converts directly to Rifts, that means this character can splat anything in the game that is not literally invulnerable per action. Then just make them a Mega-Hero and make sure they have the Major Power Supernatural Strength and Tremendous P.S. as a Mega-Power. Almost nothing in the game can resist you. Worse comes to worst, grab a stake, some silver, a holy weapon, and a few other things you can use to make sure you kill invulnerable, intangible, etc. foes. Done.
B) Zebuloid Cyber-Knight. Even without the combos people mentioned of Battle Fury Blades, Magical-Adrenal Rush, etc., you can still dump out THOUSANDS of MDC a round. At level two. With NOTHING but the RCC and OCC. Let alone adding Amaki psi-sword amplifiers or Caliber-X, etc. etc.
C) Anything with Spin at High Velocity. That move just makes everything better.
D) Atlantean Stone Master. x1000 PS for picking up rocks. Now add on Gravity Manipulation, Weight Manipulation, supernatural P.S. buffs from spells and scrolls, Stone Ox training from Japan, and eventually you can splat Apsu with a rock.
In general:
ALWAYS take Ninjas and Superspies martial arts if your class is eligible for it. Screw O.C.C. Related skills.
ALWAYS take mutant powers if you can with your O.C.C. Screw O.C.C. Related skills.
Etc.
Not by design though. And even a bladesinger competed on the same order of magnitude as other characters.
In D&D, the most heinous examples of powergameing, rulebreaking optimization require some serious work and very specific combos.
In rifts, they require choosing from a basic character class.
Wrong, wrong, wrong.
In one campaign, Goblins-themed, I rolled a Krynn Minotaur Monk. A 0 LA level 1 monk was thrashing third-to-fifth level enemies with class levels.
Anyone who rolls a Wizard, Druid or Cleric and isn't doing everything the Fighter or Barbarian can do but far, FAR better by fifth level is holding back or doesn't know how to play their class. Druids alone have class features better than entire classes.
In Rifts, lucky rolls for attributes rarely make you substantially stronger than everyone else, except for P.P. You have to get above 16 to see real benefits. In D&D, if I roll (as if I often do) a character with an 18, 18, 16, 16, 14, 12, I will make the person with an elite array a joke.
Some of the optimized combos are complex and hard to understand. Warhulk and Hulking Hurler is brain-smashingly obvious. The Vermin Lord H.I.V.E. trick is also pretty obvious. If you do the Vermin Lord trick, you can splat gods with your spells. Tome of Nine Swords combinations are so myriad and obvious it's tragic to even have to mention it. Not to mention the awful wording of Iron Heart Surge. And remember, while the differences may not be orders of magnitude, in Rifts anyone can pick up an absurdly broken energy weapon or railgun and do 2D6x10 M.D. and at least compete with the Cosmo-Knight's damage per round. But in D&D, the Barbarian and Fighter are likely to be literally useless.
Planar Shepherds have TWO, count them TWO, broken mechanics that are mind-blowingly obvious. First: Turn into extraplanars. Turn into a djinn, get Wishes, profit. Second: Manifest a bubble of the plane you're native to. Get 10 turns to their one. Profit.
Almost any PrC makes you better than almost any level of a base class, save Wizard, Druid, and Cleric.
And Rifts does not have a Pun-Pun.
I agree that, in general, Rifts is more broken. In particular, with Rifts different class choices give you different ranges of optimization, and a lot of classes are set at one level (e.g. a Dead Boy can't be optimized very much beyond sucking, and a Cosmo-Knight can't be optimized very much above or below being obscene). In other words, in D&D, the Wizard or the Druid can avoid casting the cheapest spells, specializing in the cheapest schools, and using the most broken wildshapes, but there's very little a player can do if they want to play a Cosmo-Knight but want to tone them down.
But I think you have to take Rifts more as a suite with options. You can organize it into tiers. A Cosmo-Knight, a Spirit Warrior, a Lizard Mage Mystic Knight, a Warlock (oh GOD Warlocks), a Xiticix Super-Warrior, and a converted Heroes Unlimited character can have a great time smashing face. Or you can roll a pure Coalition/Triax campaign and watch as your players have to run from the aforementioned team in mortal terror.
That having been said, IMHO, Phase World is actually fairly balanced. Heavy hitters like the Quatoria, Cosmo-Knight, Space Minotaur, etc. can co-exist pretty peacefully. C.J Carella was twinky as hell, but I will say this for most of his books: Aside from the standard skill monkey OCCs you have to throw into every Rifts book, most options within them are totally balanced.